64 Physical Science Research Frontiers 2022 With increasing demand for “green technology” to reduce CO 2 emissions, the development of highly efficient electric motors and generators is required. For this, high-performance permanent magnets , especially for traction motors of EVs, HEVs, and turbines of wind power generation, are necessary. Because the working temperature of EV/HEV motors is approximately 150°C, a high coercivity ( m 0 H c ) larger than 0.8 T is necessary at this temperature. Although Nd–Fe–B is the most important permanent magnet material for “green technology,” one drawback is its low thermal resistance to m 0 H c owing to its low Curie temperature ( T c ) [1]. Therefore, there is a strong demand for the development of new permanent magnets whose magnetic properties are superior to those of Nd–Fe–B , especially at elevated temperatures. One of the candidate materials for the next-generation permanent magnets is an RT 12 - based compound with a ThMn 12 structure, where R and T represent rare-earth and transition elements, respectively. The RT 12 -based compound is expected to show high saturation magnetization ( M s ) owing to its highest Fe composition among rare-earth magnet materials. In addition to the high M s , it has high Curie temperature ( T c ) and high anisotropy ( K ). Hirayama et al . reported that Sm(Fe 0.8 Co 0.2 ) 12 has the intrinsic magnetic properties of M s ~ 1.78 T, anisotropy field of 12 T and T c ~ 859 K, which a r e s u p e r i o r t o t h o s e o f N d 2 F e 1 4 B [ 2 ] . T h u s , S m ( F e 0 . 8 C o 0 . 2 ) 1 2 i s a t t r a c t i n g a t t e n t i o n a s a new permanent magnetic material. Since these magnets are used in various applications at various temperatures, this study was conducted to clarify the internal magnetic field of Sm(Fe 0.8 Co 0.2 ) 12 thin films and its temperature dependence. Figure 1 shows (a) magnetization curves, (b) the temperature dependence of the magnetization and (c) the temperature dependence of the normalized magnetization in Sm(Fe 1– x Co x ) 12 ( x = 0, 0.07, 0.2) thin films. All samples show strong perpendicular anisotropy. Saturation magnetization and Curie temperature increase with Co content. The thermal resistance in the saturation magnetization is the highest in the sample with x = 0.2. Figure 2 shows the (a) Mössbauer spectrum and fitting curves, and (b) temperature and (c) Co composition dependences of the Mössbauer spectrum of SmFe 12 thin films at 300 K. The 57 Fe Mössbauer spectra were measured in the temperature range of 80–523 K at SPring-8 BL11XU . The Mössbauer spectra were nicely fitted by three different Fe sites of 8i, 8j and 8f. The disappearance of 2 nd and 5 th absorptions corresponds to the strong (001) texture of SmFe 12 , which shows good agreement with the XRD result [3]. The fitting parameters are summarized in the inset. The hyperfine field decreases with increasing temperature and increases with Co content. Figure 3 shows the temperature dependence of (a) average and (b) normalized magnetic moments in Sm(Fe 1– x Co x ) 12 ( x = 0, 0.07, 0.2) thin films. These temperature dependences well match with those of the magnetization of the sample shown in Fig. 1. Figure 3(c) shows the change in hyperfine field as a function of the Fe–Fe distance. The Fe– Fe distance was estimated by XRD analysis. The magnetic moment of Fe linearly increases with the Fe–Fe distance, and the effect is similar for all Co Magnetic moments of Fe site in ThMn 12-type Sm(Fe 1– x Co x ) 12 compounds and their temperature dependence Fig. 1. (a) Magnetization curves of Sm(Fe 1− x Co x ) 12 thin films ( x = 0, 0.07, 0.2) measured perpendicular and parallel to film plane at 300 K, (b) temperature dependence of spontaneous magnetization, m 0 M s , and (c) normalized saturation magnetization m 0 M s / m 0 M s (0) versus normalized temperature T / T C of Sm(Fe 1− x Co x ) 12 ( x = 0, 0.07, 0.2) thin films. [3] (a) (b) (c) H (T) T (K) T / T C M (T) M s (T) M s (T)/ M s (0) 0 0 0.0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 0.2 0.4 0.6 0.8 1.0 1.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 200 400 600 800 0.0 0.5 1.0 1.5 2.0 300 K 300 K 2 4 6 Co 0% ⊥ Co 7% ⊥ Co 20% ⊥ Co 0% Co 7% Co 20% Co 0% // Co 7% // Co 20% // Co 0% Co 7% Co 20% 65 Research Frontiers 2022 contents; the interdependence of the Fe–Fe distance and Fe magnetic moment is caused by the magneto- volume effect associated with d -orbital electrons. The increase in Fe magnetic moment upon Co addition is independent of the Fe–Fe interstitial distance and is similar to the trend shown in Fig. 1. In conclusion, we first investigated the dependences of temperature and Co composition on the magnetic moment of crystallographically different Fe sites in Sm(Fe 1– x Co x ) 12 ( x = 0, 0.07, 0.2) thin films by in situ synchrotron Mössbauer spectroscopy. The microscopic origin of the magnetism and temperature dependence of Sm(Fe 1– x Co x ) 12 ( x = 0, 0.07, 0.2) thin films is clarified by experiments and theoretical calculations. Fig. 2. (a) Mössbauer spectrum of SmFe 12 thin films measured at 300 K with fitting curves using three independent spectra for 8i, 8j, and 8f Fe sites, (b) temperature dependence of Mössbauer spectrum of SmFe 12 thin films, and (c) Co content dependence of Mössbauer spectrum of Sm(Fe 1− x Co x ) 12 thin films at 300 K. The dash line and arrows in (b) show the peak positions of 1 st and 6 th lines at each temperature. [3] Fig. 3. (a) Temperature dependence of magnetic moments at various Co contents and (b) normalized magnetic moments at various Co contents site versus normalized temperature for Sm(Fe 1– x Co x ) 12 thin films ( x = 0, 0.07, 0.2). (c) Calculated results of normalized magnetic moments at each Fe site versus reduced temperature for Sm(Fe 1– x Co x ) 12 compounds ( x = 0, 0.07, 0.2). Yukiko K. Takahashi National Institute for Materials Science Email: TAKAHASHI.Yukiko@nims.go.jp References [1] M. Sagawa et al .: J. Appl. Phys. 55 (1984) 2083. [2] Y. Hirayama et al. : Scr. Mater. 138 (2017) 62. [3] D. Ogawa, T. Fukazawa, S. Li, T. Ueno, S. Sakai, T. Mitsui, T. Miyake, S. Okamoto, S. Hirosawa, Y. K. Takahashi: J. Magn. Magn. Mater. 552 (2022) 169188 . (a) (b) (c) Velocity (mm/s) 4 8 0 0.9 1.0 Co 0%, 30 Co 0%, 300 0K K – 4 – 8 Velocity (mm/s) 4 8 0 – 4 – 8 Velocity (mm/s) 4 8 0 – 4 – 8 Relative Transemission 0.9 0.95 0.9 1.0 0.9 1.0 1.00 1.0 0.9 1.0 Exp. Fit. 8i 8j 8f 0.9 1.0 Relative Transemission Relative Intensity Co 0%, 8 Co 0%, 80 0K K Co 0%, 15 Co 0%, 150 0K K Co 0%, 30 Co 0%, 300 0K K Co 0%, 30 Co 0%, 300 0K K Co 7%, 30 Co 7%, 300 0K K Co 20%, 30 Co 20%, 300 0K K z B y Isomer shift (mm/s) Hyperfine field (T) Q shift (mm/s) 8i value error value error value error –0.0597 0.01467 –0.1581 0.02593 –0.194 0.02055 32.323 0.03508 29.225 0.03904 27.415 0.05191 0.106 0.02933 –0.077 0.04803 –0.107 0.03812 8j 8f (a) (b) (c) T (K) T / T C d Fe – Fe (nm) 0 0.0 0.0 0.2 0.4 10 15 1.0 1.5 2.0 2.5 20 25 30 35 40 0.6 0.8 1.0 1.2 0.5 1.0 1.5 2.0 2.5 200 400 600 800 0.0 0.2 0.4 0.6 0.8 1.0 1.2 0.250 0.260 0.270 Average m Fe ( μ μ B ) Normalized m Fe Hyperfine Field (T) μ μ Co 0% Co 7% Co 20% Co 0% Co 7% Co 20% Co 0% Co 7% Co 20% Sm(Fe 0.75 Co 0.25 ) 11.5 Ti 0.5 in Ref. 25 300 K 300 K 8f 8j 8i Magnetic Moment ( B )
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